There isn't a way to override properties in a partial class, you need to do it in a derived one. However using a derived class might not help, given that the OpenAccess context will retrieve instances from the base class and there isn't an easy way of converting those to your new types.
What you can do is add a property with a different name (in the partial class), that does the neccessary calculations. This however will mean that you will have both exposed on the model. In order to fix that you can change the access modifier of the generated property trough the visual designer. Just find the property and change it's access modifier in the properties pane to something different from public.
I will post this answer on your stack overflow thread as well, so that it is available to the community. I would like to also mention that posting questions there is a nice way to provide solutions to problem to a more broader audience.
All the best,
the Telerik team
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